According to Woody Allen, it is his second favourite organ and it absorbs more than 25% of the energy that our bodies generate. But why? For what purposes did the human brain evolve and why does it take so much of our physiological resources? Such questions have absorbed scientists for decades and have now been given an expected answer by Colin Blakemore. In a recent lecture, the Oxford neurobiologist argued that a mutation in the brain of a single human being 200,000 years ago turned intellectually able apemen into a super-intelligent species that would conquer the world. In short, Homo sapiens is a genetic accident.

Most scientists believe we achieved our intellectual status through gradual evolution. Blakemore's intervention will therefore come as a surprise and an upset, although this will not faze the provocative 66-year-old.

So why have you decided to put forward your idea now?

I was asked to give the Ferrier prize lecture at the Royal Society and as this is the society's 350th anniversary I decided to do something special and face up to the issue of the human brain. The question is: why is it so big compared to the brains of our predecessors, such as Homo erectus? Until 200,000 years ago, there had been a gradual increase in brain size among hominins, starting three million years ago. Then, abruptly, there was a remarkable increase of about 30% or so.

How have scientists explained this jump in brain size?

Many have argued that if there was a dramatic increase in brain size, there must have been a fantastic advantage that came with it: improvements in tool construction, more complex language and other cultural changes. In other words, they say simple natural selection explains what happened.

So what is your take on this view?

I think they're fooling themselves. There was very little change in human behaviour at this time, as far as we can see from the fossil record – certainly not one that is explained by a sudden jump in the size of the human brain. These hand-waving arguments about tiny changes in culture explaining the emergence of such a huge change in brain structure just doesn't hold water. It's like arguing that a reptile suddenly developed fully formed wings and then sat around for 200,000 years before suddenly saying: Oh my God, I've discovered I can fly. It's ridiculous.

So what did happen?

Genetic studies suggest every living human can be traced back to a single woman called "Mitochondrial Eve" who lived about 200,000 years ago. My suggestion is that the sudden expansion of the brain 200,000 years ago was a dramatic spontaneous mutation in the brain of Mitochondrial Eve or a relative which then spread through the species. Achange in a single gene would have been enough.

What effect did this have?

Very little at first. The environment of early humans was so clement and rich in resources that this greedy new brain, which would have absorbed even more of the body's energy, could be sustained without danger. Later, when times got hard, during droughts or climate changes, it helped us deal with these crises, which could otherwise have killed us off, by dreaming up novel ideas to problems.

What are the implications of this idea?

My argument stresses the plasticity that our brains were endowed with when this mutation occurred. Some scientists believe that skills like language have a strong genetic basis, but my theory stresses the opposite, that knowledge, picked up by our now powerful brains, is the crucial mental component. It means that we are uniquely gifted in our ability to learn from experience and to pass this on to future generations. That has a bad side: a single generation starved of knowledge, thanks to some global disaster, for example, would be cast back to the Stone Age. Everything would be undone. On the other hand, there is no sign that the human brain has reached its capacity to accumulate knowledge, which means that the wonders we have already created – from spaceships to computers – represent only the start of our achievements.

In fact, we're getting awfully good at developing technologies that remove functions from us, the sort of stuff that used to be done by brains – calculating and remembering things, for instance. Before the invention of reading and writing, people's memories would have been much better developed than our own. I knew people at school who could churn out masses and masses of Shakespeare. No young kid could do that now. But they don't need to. And you have to ask: "Well, what else are they doing with their brains that used to do that kind of stuff?" I'm pretty sure that other things are being done...